1. Field of the Invention
The present invention relates to a sheet feeding device for feeding a sheet by loosening sheets by blowing air to a sheet stack and an image forming apparatus including the sheet feeding device.
2. Description of the Related Art
As one of methods for feeding sheets one by one from sheet storage means (hereinafter referred to as storage compartment) in an image forming apparatus such a printer, a copying machine, conventionally, an air sheet feeding device has been proposed. According to this method, as disclosed in Japanese Patent Application Laid-Open NO. 9-309624, gas (mainly, air) is blown to an end portion of a sheet stack to lift up plural sheets and separate them from each other and the sheet is carried while sucked with a conveyance belt.
An example of the conventional air sheet feeding device will be described with reference to FIGS. 12, 13. FIG. 12 is a schematic sectional explanatory diagram of the conventional air sheet feeding device and FIG. 13 is a sectional view taken along N-N in FIG. 10.
A storage compartment 201 includes a sheet tray 202 which is capable of lifting up/down. A sheet stack S is placed on this sheet tray 202 and side restricting plates 203a, 203b are struck against both end portions in the width direction (direction perpendicular to the sheet feeding direction) of the sheets. Further, a rear end restricting plate 204 is struck against the rear end of the sheets. Consequently, the sheet stack S placed on the sheet tray 202 is positioned.
Upon feeding the sheets, the sheet tray 202 is raised by driving means (not shown) and stopped at a position in which the topside of the sheet stack S is off a suction belt 205 by a predetermined distance. Then, when a separation fan 206 is actuated to blow air to the side face at the topside of the sheet stack, several pieces at the topside of the sheet stack are loosened. With this condition, a suction fan 207 is actuated to suck the sheet at the topmost to the suction belt 205 and that belt 205 is rotated to feed a single sheet.
When for example, 2000 pieces of sheets having a weight of 80 g/m2 are accommodated in the above-described air sheet feeding device, the height thereof before floated, that is, C in FIG. 13 is about 220 mm. It is very difficult for user to set these accurately without any deviation in the sheet width direction. Thus, there has been proposed a device adopting the structure which allows right and left side restricting plates 203a, 203b to move interlockingly in the restricting direction with respect to the center of the sheet setting position so as to restrict the position of the sheets securely.
In such a device, first, the positions of the side restricting plates 203a, 203b are adjusted corresponding to the size of a small amount of sheets taking care so that there is no gap between the sheet stack S and the side restricting plates. If remaining sheets S are set with this condition, no deviation in the sheet width direction occurs.
However if the sheets are set so strictly that there is no gap between the sheet stack S and the side restricting plates, a large friction resistance is generated between the surfaces of the side restricting plates 203a, 203b and the sheet end portion in a portion D of FIG. 13. As a result, when air is blown against a sheet, it might not be floated sufficiently enough to be fed. Further, if the dispersion of the size of each sheet is large or the dispersion of the dimension of the side restricting plates 203a, 203b or other component is large, tendency of sheet feeding failure becomes more conceivable.
The air sheet feeding device is a system susceptible to an influence of friction resistance at the portion D in FIG. 13 described above. If the quantity of blown air is increased to eliminate this tendency, the size of the air supply means is enlarged or the quantity thereof is increased, thereby leading to enlargement of the size of the entire apparatus or increase of manufacturing cost of the product.